I. Field of the Invention
This invention relates generally to computer data input devices, and more particularly to a cordless light pen system for inputting data to a computer corresponding to graphic patterns presented on the face of a cathode ray tube display.
II. Discussion of the Prior Art
In the field of computer-aided design or in software development, the designer or programmer commonly works with a keyboard and a graphics display terminal to form machine part patterns or programming data on the screen of a cathode ray tube. During the design procedure, it becomes necessary to send messages to a computer as to the dimensions and shapes of the object being designed or, in the case of software, to indicate a selected menu item or the like. The computer can then, in turn, prepare a tape which may later be used by a tape reader-controlled machine tool to produce the part. Similarly, in the case of software development, programmers often have a need to communicate interactively with data presented on the CRT monitor.
Because presenting data to a computer from a keyboard in certain applications is a time consuming approach, so-called "light pens" have been developed for facilitating the data entry operations.
As those skilled in the art understanding, in a CRT, the electron beam generally has its Home position in the upper left corner of the screen, and time varying voltages are applied to the deflection plates or to the deflection magnets to cause the electron beam to sweep from left to right until a horizontal sync pulse is encountered. At this time, the beam is blanked and quickly returns to the left edge of the screen just below the starting point of the line it had just previously traced. This continues until a vertical sync pulse is encountered, generally when the sweep is at the lower left corner of the screen. The vertical sync pulse defines the end of a frame. Upon encountering the vertical sync pulse, the beam is again blanked and moves rapidly to the starting point at the upper left corner of the screen where the sequence is again repeated. In digital display apparatus, each horizontal line consists of a series of dots called picture elements or pixels. In a typical display, each line may consist of 480 pixels and there may be 240 lines for each frame with the sweep speed being such that 60 frames are traced each second.
The redrawing of the CRT screen is called refresh. A special address calls out the coordinates which tell the beam where to move. This same refresh addressing also affects addressing of the memory locations which store the data specifying color and dot information.
Whether a given dot on the screen will be bright or dark depends upon the contents of a random access memory forming a part of the character generating circuitry. Associated with the random access memory are addessing circuits consisting of a vertical counter and a horizontal counter, the counters being appropriately driven by a clock circuit termed the "dot clock". Thus, as the horizontal and vertical counters are advanced, the contents of the RAM are sequentially read out and used to either blank or unblank the electron beam, depending upon the information stored in the RAM. As the scanning line refreshes a given point on the phospher during its travel across the screen, that point brightens momentarily.
When a light pen is positioned adjacent the CRT screen, this momentary increase in brightness of the light emitted by the screen's phospher within the light pen's view of field causes a change in signal current from the photo-detector within the light pen and this signal is amplified, filtered and applied to a comparator where it is compared against a predetermined threshold voltage. If the threshold is exceeded, a "hit" pulse is produced. This hit pulse latches the current screen refresh address in a register. Thus, knowing the address where the hit occurred, the computer may also determine the coordinate location of the pen on the face of the screen.
In the prior art systems, the "hit" signals are fed through an electrical cord coupling the light pen to an input port of a computer or computer interface device and may cause a time or count value to be captured in a snap-shot register within the computer.
One reason why light pens have not been widely adopted as a user/monitor interface device has been due to some extent to operator fatigue. When using the light pen, the user must position the point of the light pen against a usually vertical screen and without any form of arm support. The weight of the pen, its cord and the spring tension of the cord adds to this operator fatigue and thus detracts from its value as a pointer.
The present invention is considered to be an improvement over the prior art in that it obviates the fatigue-inducing cord. Rather than sending electrical signals from the light pen to the computer by way of the conductors in the cord, the same information is transmitted from the light pen to a receiver connected to the computer by light energy. In addition, the design of the light pen of the present invention has been engineered to reduce its weight while still having the pen easy to grasp and easy to position and operate.
The cordless light pen of the present invention also provides certain other benefits over prior art light pens which are coupled by a cord to the computer terminal with which they are used. More specifically, a light pen having a cord is often subject to electrostatic discharge (ESD), either from an operator upon first picking up the light pen or from the charge which may be present on the CRT screen itself. Such ESD can cause the computer to interpret the discharge impulse as a valid "hit" signal or, alternatively, the static charge itself may travel through the cable and damage sensitive components in the computer. By eliminating the need for the cord, both of these potential problems are eliminated.
In addition to curing the ESD problem, the cordless light pen of the present invention also removes problems occasioned by electromagnetic interference (EMI). The emission of EMI by electronics systems is now tightly regulated by federal law. Computers with high frequency clocks are notorious for the generation of EMI and, hence, their enclosures must be carefully shielded and other noise reducing measures must be taken to minimize these emissions from CRT terminals. The long cable of the prior art light pens frequently act as antennas, causing a terminal to fail the applicable emission standards. By eliminating the cord on the light pen, the federal standards can more readily be met while still allowing a light pen instrument to be used as a data input device with the system.
Finally, simple mechanical unreliability associated with a cable is absent in the cordless light pen system of the present invention. As is well known in the art, cords attached to manipulated objects are subject to many mechanical stresses, torques and other forces. This often leads to breakage of internal conductors and, thus, may frequently cause intermittent faults for total lack of operation.